Inaccuracies caused by molecular collision broadening, manifest themselves as span effects. That is, the spectral-line shape identifying absorption of infrared radiation by a subject fluid is altered. This effect is described by Jamieson, McFee, Plass, Grube, and Richards in Infrared Physics and Engineering, 1963 published by McGraw-Hill, pages 32-37. The effect as it relates to anesthetic gases is described by D.L. Hill and T. Powell, J. Sci. Instrum., 1967 Vol. 44, pages 189-194.
In the course of development of an infrared analyzer to determine carbon dioxide in breath, this effect was prevalent when an anesthetic gas, for instance, nitrous oxide, was present. Nitrous oxide (N.sub.2 O) is the anesthetic gas of greatest concern because of the high concentrations used on a patient. As great a concentration as 80% nitrous oxide in oxygen is used.
When, for example, a typical infrared analyzer is calibrated on 10% carbon dioxide in air, and the nitrogen present is replaced with nitrous oxide, collision broadening of the carbon dioxide molecule takes place. That is, if 10% carbon dioxide in 10% oxygen and 80% nitrous oxide is passed through the analyzer, the analyzer only reads 9.5% carbon dioxide, as shown in FIG. 5. This effect is most prevalent in highly selective analyzers. These analyzers are of the type described by Hill, or further described in U.S. Pat. No. 4,598,201 to Fertig, et al. The effect also is manifested in analyzers having highly selective infrared sources such as laser sources where discrete lines of energy coincide with the absorption lines of the fluid of interest, or the selective type sources described in U.S. Pat. No. 2,212,211 to Pfund.